1. Field of the Invention
The present invention relates to a slotting apparatus for producing a slot in the inner circumferential surfaces of a tubular workpiece such as a sleeve for a rotary valve for use in a power steering system for an automobile.
2. Description of the Background Art
FIG. 12 of the accompanying drawings shows a sleeve W for use in a rotary valve, which has a plurality of angularly spaced port slots a defined in the inner circumferential surface thereof, each having a substantially arcuate cross section in the longitudinal axial direction of the sleeve W. Such port slots a can be produced by a slotting apparatus as disclosed in Japanese patent publication No. 49-49541, for example.
The disclosed slotting apparatus has a slotting tool having a cutter on its distal end. To produce port slots a, the sleeve W is swingably supported on the slotting tool, and then the slotting tool is inserted into the sleeve W. While the sleeve W and the slotting tool are being angularly moved, the cutter progressively cuts the inner circumferential surface of the sleeve W at a desired location on an intermediate region of the inner circumferential surface, thus forming a port slot a.
With the conventional slotting apparatus, however, since the sleeve W and the slotting tool have to be angularly moved in appropriately timed relationship to each other, mechanisms for angularly moving the sleeve W and the slotting tool are relatively complex, large, and expensive. It is also difficult to angularly move the sleeve W and the slotting tool at high speed while keeping them in appropriately timed relationship to each other. Therefore, the prior slotting apparatus has suffered difficulty in slotting the workpiece efficiently within a short period of time.
Another slotting apparatus is disclosed in Japanese laid-open patent publication No. 62-34714, for example.
The slotting apparatus disclosed in the latter publication includes a crank arm having a pair of upper and lower cutters on its distal end, and a crankshaft from which the crank arm extends, the crank arm being swingably supported at its intermediate portion. For producing port slots a in the sleeve W, the cutters are inserted into the sleeve W that is held in position, and then the crankshaft is angularly moved to reciprocally move the cutters together with the crank arm substantially in the axial direction of the sleeve W while at the same time angularly moving the cutters in the vertical direction. The cutters are thus caused to follow substantially elliptical paths in phase with each other. At this time, the cutters cut the inner circumferential surface of the sleeve W in first half strokes of the substantially elliptical paths, thereby simultaneously producing a pair of upper and lower port slots a in the inner circumferential surface of the sleeve W.
The above slotting apparatus is capable of simultaneously producing a pair of port slots a in the sleeve W. However, inasmuch as it is generally difficult to attach both cutters to the crank arm in highly accurate positional relationship, the cutters tend to describe slightly different paths during the slotting process, and hence the port slots a cannot easily be produced to identical shape with high precision.
In addition, because the cutters follow substantially elliptical paths, when the cutters return to their original positions in next half strokes of the paths after they have cut the inner circumferential surface of the sleeve W in the first half strokes, the cutters and the distal end of the crank arm on which they are mounted are displaced from the slotted region relatively largely toward the axis of the sleeve W. Such a cutter displacement poses a problem on attempts to increase the rigidity of the crank arm by increasing the diameter of the distal end of the crank arm for greater resistance to high loads applied to the cutters when they slot the sleeve W. Specifically, the diameter of the distal end of the crank arm cannot be increased to a large extent because the crank arm and the sleeve W must be kept out of physical interference with each other. As a consequence, it has been difficult to give the crank arm a sufficient degree of rigidity to withstand high loads imposed on the cutters during the slotting process.
Japanese patent publication No. 63-47565 also discloses still another slotting apparatus. The disclosed slotting apparatus includes a crank arm having a cutter on its distal end and extending from a crankshaft, the crank arm being swingably supported on its intermediate portion. The cutter is caused to follow a predetermined path to produce a port slot a in the sleeve W by a cam on the crankshaft and a cam follower held against the cam when the crankshaft is angularly moved. At the same time, the cutter is also caused to move toward and away from the inner circumferential surface of the sleeve W by a second cam that is held against the cam follower through a rod, for slotting the inner circumferential surface of the sleeve W.
The above slotting apparatus is however disadvantageous in that the two cams, which are relatively large in size, are juxtaposed on one plane, and need respective complex drive mechanisms, thus making the slotting apparatus large and complex.